I have a conventionally built attic insulated with R-30 batts. I have been given enough 2 3/4" polyiso 4 x 8 panels to cover my entire attic with 2 layers of this foam over the joists. Is there any problem with doing this? Is there a better way to use the foam panels without major retrofit? The house is in Arkansas.

It shouldn't be any issue with laying the polyiso on top of the fiberglass bats, especially if it is free; however, you need to determine if you might be creating a moisture trap. What kind of facer does the polyiso have? Do your batts have a vapor barrier on the side facing the ceiling? Is your roof constructed with trusses or stick built? Do you have adequate roof and soffit ventilation?

What about the gaps between the fiberglass bats? You might want to consider a light overspray of ~3" of loose cellulose to fill those gaps between the batts to better air seal the attic floor before you lay down the polyiso.

I'm currently building a new home in Arkansas - roofing felt is on the deck now and shingles are scheduled to be installed Monday. I've been trying to find some of those reclaimed polyiso sheets to rip and lay down before I have cellulose blown in the attic, but haven't had any luck finding anything in the area. I need around 2200 square feet (~70 4x8 sheets). Does your source for the polyiso sheets have any more they would be willing to sell?

Thanks,

My email is arkie6 at yahoo dot com

The panels are fiberglass/felt faced.   The batts do have a vapor barrier next to the drywall.   Roof is trussed but with room for panels.   I have very good ventilation soffit and ridge vent

The light overspray of cellulose sound like a good idea to fill the void.   The insulation guy says I can only go about 10 inches over the batts before settling  becomes an issue.   The foam panels would alleviate the problem.

There is a guy on craigslist out of ST. Louis that sells these panels in large quantities.    Also these panels are very nasty to work with when you start cutting them.